The present invention relates generally to cathode ray tube beam deflection circuits and more particularly to beam deflection circuits which permit both linear and resonant nonlinear operating modes.
Linear amplifiers are able to deflect the electron beam in a cathode ray tube (CRT) with great precision in response to complex input drive signals. Hence, linear deflection amplifiers are capable of being used to display a variety of scan formats including stroke, dot and raster. The chief disadvantage of linear deflection amplifiers is the extremely high power required to perform the rapid retrace with raster scan formats.
Resonant flyback amplifiers are widely used in raster scan displays due to their inherent simplicity and efficiency in rapidly retracing the electron beam. However, resonant flyback deflection amplifiers have relatively poor linearity and are thus limited to the raster scan format.
In the prior art, linear and resonant flyback or nonlinear deflection have been combined to provide the benefits of both types of CRT beam deflection. For example, Spilsbury U.S. Pat. No. 4,297,621 discloses a dual mode cathode ray beam deflection circuit. In Spilsbury, the deflection coil of a CRT is connected as the load of a push/pull amplifier. The negative half of the push/pull amplifier is connected to the deflection coil by means of a field effect transistor (FET) and the postive half of the push/pull amplifier is connected through a capacitor which is bypassed by a diode during linear operation of the deflection circuit.
For resonant flyback or nonlinear operation of the Spilsbury deflection circuit, a flyback drive pulse from an external generator is applied through a transformer to bias the FET into a nonconducting state such that the negative half of the push/pull amplifier is abruptly disconnected from the deflection coil. The diode bypassing the capacitor is reverse biased such that the deflection coil resonates with the capacitor connected to the positive half of the amplifier and is connected to a positive power supply through the capacitor and a second diode to effect rapid retrace.
Since the deflection coil is disconnected from the negative half of the push/pull amplifier in Spilsbury, the push/pull amplifier cannot be used to enhance resonant flyback or nonlinear operation. Further, externally generated drive pulses must be provided to change the deflection system from linear to resonant nonlinear operation and resonant or nonlinear operation can be effected in only one direction.
It is thus apparent that the need exists for an improved cathode ray tube beam deflection circuit which permits dual mode operation in both linear and resonant nonlinear modes without disconnection of a driving amplifier during resonant nonlinear operation, provides for internal switching between the two modes of operation and can provide for bidirectional resonant nonlinear operation to accommodate complex beam driving signals which include rapid changes or beam retrace in either direction.